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Seasonal effects on stream community structures

Think about your garden, where you might plant vegetables in late spring and expect to harvest them throughout the summer. Fertilizers are often required to provide nutrients to the plants, and pests like insects might consume the plants before you can harvest them. Just like plants in a garden, algae in streams tend to increase during the summer growing season, especially in streams that are full of nutrients. Similarly, aquatic insects consume algae and keep abundances low, or even change the community composition of the algae.

Seasonality has drastic impacts on stream algae because nutrients and aquatic insects can change over time. However, this has rarely been investigated under natural conditions. In this study, we examined algal abundance and community composition over time in a mountain stream. We measured and manipulated factors like temperature and nutrients that promote algal growth, as well as factors like aquatic insects that tend to decrease algae. For a series of three experiments from August to October, we used small vials capped with discs (i.e., algal growth surfaces) to add nutrients and an underwater electric fence to exclude insects.

This study examined how seasonality influenced algae in a Colorado mountain stream. Shown here are photos taken in September (left) and October (right) during the study. (Photo credits: Whitney Beck)

Interestingly, we found that nutrients strongly stimulated algal growth earlier in the season when temperatures were warm, but nutrient additions had no effect on algae when temperatures cooled. We also found that aquatic insects decreased algal abundances and changed algal community composition early in the study. However, later in the season, the dominant insect consumer emerged from the stream into a flying adult stage. With fewer insect consumers in the stream, insects had little influence on the algae.

Our results have important implications for ecological studies; if researchers complete an experiment or sample an ecological community one week, they might find different results the next week because of natural changes that are occurring. This happens in your garden too, where seasonality influences the soil nutrients and much how fertilizer is required. Seasonality also dictates when insect pests will arrive and plague the plants. Our results may also be important for managers who are interested in controlling algae to improve water quality, as frequent monitoring may sometimes be necessary to account for seasonal changes.

We constructed an underwater electric fence to exclude aquatic insects from consuming algae in streambed plots. We then compared these plots to non-electrified plots, where insects were allowed to graze freely on the algae. This allowed us to determine the effect of insect consumption on algal abundance and community composition. Within each plot, we also added nutrients using small vials. (Photo credits: Whitney Beck)

Oikos Journal

Oikos is a journal issued by the Nordic Ecological Society and is one of the leading peer-reviewed journals in ecology. Oikos publishes original and innovative research on all aspects of ecology, defined as organism-environment interactions.